Light-emitting devices may be generally divided into organic light-emitting devices in which a light-emitting layer is formed from an organic material and inorganic light-emitting devices in which a light-emitting layer is formed from an inorganic material. Organic light-emitting diodes (OLEDs), a component of organic light-emitting devices, are self-emitting light sources based on the radiative decay of excitons in an organic light-emitting layer, the excitons being generated by the recombination of electrons injected through an electron injection electrode (cathode) and holes injected through a hole injection electrode (anode). OLEDs have a range of merits, such as low-voltage driving, self-emission, a wide viewing angle, high resolution, natural color reproducibility, and rapid response times.
Recently, research has been actively undertaken in order to apply OLEDs to a variety of devices, such as portable information devices, cameras, watches, office equipment, vehicle information display devices, televisions (TVs), display devices, illumination systems, and the like.
In order to improve the luminous efficiency of OLEDs, it is necessary to improve the luminous efficiency of a material that constitutes a light-emitting layer or to improve light extraction efficiency in terms of a level at which light generated by the light-emitting layer is extracted.
Here, light extraction efficiency depends on the refractive indices of the layers of materials that constitute an OLED. In a typical OLED, when a beam of light generated by the light-emitting layer is emitted at an angle greater than a critical angle, the beam of light may be totally reflected at the interface between a higher-refractivity layer, such as a transparent electrode layer, and a lower-refractivity layer, such as a glass substrate. This consequently lowers light extraction efficiency, thereby lowering the overall luminous efficiency of the OLED, which is problematic.
More specifically, only about 20% of light generated by an OLED is emitted outwards and about 80% of the light generated is lost due to a waveguide effect originating from the difference in refractive indices between a glass substrate and an organic light-emitting layer that includes an anode, a hole injection layer (HIL), a hole transporting layer (HTL), an emission layer (EML), an electron transporting layer (ETL), and an electron injection layer (EIL), as well as by the total internal reflection originating from the difference in refractive indices between the glass substrate and the ambient air. Here, the refractive index of the internal organic light-emitting layer ranges from 1.7 to 1.8, whereas the refractive index of indium tin oxide (ITO), generally used for the anode, is about 1.9. Since the two layers have a significantly low thickness, ranging from 200 nm to 400 nm, and the refractive index of the glass used for the glass substrate is about 1.5, a planar waveguide is thereby formed inside the OLED. It is estimated that the ratio of the light lost in the internal waveguide mode due to the above-described reason is about 45%. In addition, since the refractive index of the glass substrate is about 1.5 and the refractive index of the ambient air is 1.0, when light exits the interior of the glass substrate, a beam of the light having an angle of incidence greater than a critical angle is totally reflected and trapped inside the glass substrate. The ratio of the trapped light is commonly about 35%, and only about 20% of generated light is emitted outwards.
In order to overcome the above-described problems, several approaches for improving light extraction efficiency have been examined. For example, an antireflection film may be formed by disposing a planarization layer having an intermediate refractive index between a glass substrate and a light-emitting structure, or a barrier rib may be formed as a waveguide by dispersing white particulates or transparent particulates in transparent polymer disposed on a substrate, the refractive index of the transparent particulates being different from that of the polymer.
However, even if the above-described approaches have been introduced, the light-absorbing characteristics of materials of OLEDs limit distances that light can travel forwards. Thus, light emitted by an OLED does not have sufficient opportunity to reach the light extraction layer that is disposed on the path along which the light is emitted outwards in order to improve light extraction efficiency. In addition, in the case of the barrier rib, current may be concentrated in a portion abutting the corner thereof, since the cross-section of the barrier rib is oblong. It is highly probable that a current leakage may occur, which is problematic.